This invention relates to a servo control system for controlling driving of a servo motor and, more particularly, to a servo control system of this type in which a servo driver is divided into a current control section and a position control section.
A servo motor is used for positioning of shafts in a numerical control device or a positioning device for controlling a machine tool or an automation device.
For controlling the servo motor, a conventional servo motor is provided with a servo driver of a rated output corresponding to the servo motor and a position control section for controlling this servo driver. In the following description, these servo driver and position control section will be called servo control means and a including the servo motor and the servo control means will be called a servo control system.
Referring to FIG. 9, a prior art servo control system will be described.
A servo motor 7 is, for example, an AC servo motor of an ac drive type. To the servo motor 7 is coupled a position sensor 8 for detecting a current position of the servo motor 7 and a velocity sensor (pulse generator) 9 for detecting a current velocity. Outputs P1 and P5 of these sensors are converted to position data P2 and velocity data F4 by a position sensor conversion circuit 64 and a velocity sensor conversion circuit (F/V converter) 63 and then supplied to a position control section 51 and a velocity control section 61. Further, in a case where a synchronizing type servo motor is used as the servo motor 7, the position sensor conversion circuit 64 provides to a current control section 62 a phase signal P3 for controlling switching position of a field magnetic system produced on the basis of the output P1 of the position sensor 8.
A position control system 5 comprises a position control section 51 and a D/A converter 52. The position control section 51 receives position command data F0 representing a target position of the servo motor 7 from an unillustrated high order controller and also receives the position data P2 representing the current position of the servo motor 7. The position control section 51 computes the difference between the position command data F0 and the position data P2 and provides a velocity command signal F1 corresponding to the position difference to the D/A converter 52. The D/A converter 52 converts the velocity command signal F1 from the position control section 51 to an analog velocity command signal F3 and supplies it to a servo driver 6.
The servo driver 6 comprises a velocity control section 61, a current control section 62, a velocity sensor conversion circuit 63 and a position sensor 64.
The velocity control section 61 receives the velocity command signal F3 from the D/A converter 52 and a velocity signal F4 representing a current velocity of the servo motor 7. The velocity control section 61 computes the difference between the velocity command signal F3 and the velocity signal F4 and supplies a torque signal (current command signal) T1 for the servo motor 7 corresponding to the velocity difference to the current control section 62.
The current control section 62 drives a power transistor by a three-phase PWM signal and thereby supplies a drive current of each phase (i.e., U phase, V phase and W phase) to the servo motor 7. At this time, current feedback signals T3 of the U phase and the V phase are fed back to the current control section 62 by a current detection isolator CT. The current control section 62 amplifies differences between the torque signals (current command signals) T1 of the respective phases and the feedback signals T3 of the respective phases and thereby supplies a drive current to the servo motor 7.
In the servo control system described above, the servo motor 7 must be provided with the servo driver 6 of a rated output corresponding to the servo motor 7 and, accordingly, when the servo motor 7 is replaced, the servo driver 6 must also be replaced in accordance with the output rated data of the servo motor 7. In this case, in a servo motor in which the position control system 5 is provided integrally with the servo driver 6, not only the servo driver 6 but also the position control system 5 must also be replaced.
Further, since the position control system 5 and the servo driver 6 are generally provided for each servo motor 7, an individual position control system 5 and an individual servo driver 6 are provided for a servo motor of each shaft even when it is unnecessary to control plural shafts (servo motors) simultaneously. This is a waste of costs and, besides, requires a large system.
These problems can be overcome by separating the position control system 5 from the servo driver 6.
In the above described servo control system, however, separation of the position control system 5 from the servo driver 6 can only be realized by detachably connecting them by means of detachable cables. This manner of separation, however, is not applicable to devices between which the number and size of cable connectors are incompatible and hence cannot be applied universally.
From the point of view of universal application, connection by a standardized communication line such as RS232C is preferable to connection by means of detachable cables. Since, however, such communication line is adapted to unilateral communication, it can supply a velocity command signal to the servo driver side only and necessary information on the servo driver side, e.g., rated data and disorder data, cannot be supplied to the position control system 5 and further to the high order controller side. Therefore, it is not possible for servo motors of different rated data to share the position control system 5 commonly or monitor the state of the servo motor and, accordingly, the benefits of separation of the position control system 5 and the servo driver 6 cannot be obtained. Mere separation therefore is not of much significance.
Moreover, even when the position control system 5 can be separated from the servo driver 6 (velocity control section 61), separation of them with a large distance therebetween is not preferable because it results in separation of similar control systems. On the other hand, while it is desirable to dispose the current control section 62 near the servo motor 7 since the current control section 62 controls a high voltage current compared with other control systems, positioning of the servo driver 6 having the velocity control section 61 near the device such as servo motor 7 tends to cause the velocity control section 61 to be adversely affected by noise. In the conventional system, therefore, the servo driver 6 and the servo motor 7 are provided with a suitable distance therebetween.
It is, therefore, not desirable for the construction of the conventional servo control system to separate the position control system 5 from the velocity control section 61 and dispose the servo driver 6 and the servo motor 7 with a too large distance therebetween.